The label distribution protocol (LDP) is a protocol used to distribute labels in a multiprotocol label switching (MPLS) environment. LDP relies on the underlying routing information to forward label packets. LDP is used for signaling best-effort label switched paths (LSPs). LDP has no traffic engineering capability. In other words, LDP has no concept of bandwidth or congestion.
The industry is gradually moving towards the direction of converged packetized optical networks using MPLS transport profile (MPLS-TP), such as generalized MPLS (GMPLS). MPLS-TP uses the resource reservation protocol-traffic engineering (RSVP-TE) signaling protocol to set up LSPs. MPLS-TP LSPs are traffic engineering capable (e.g., include concepts of bandwidth and congestion). MPLS-TP LSPs are the transport layer LSPs and carry all types of traffic, including upper layers, such as MPLS RSVP-TE traffic (which is traffic engineering capable) and LDP traffic (which is not traffic engineering capable). It is common for a single MPLS-TP LSP to carry both types of upper layer traffic simultaneously.
The MPLS-TP LSP has finite bandwidth, but the traffic carried on the MPLS-TP can change throughput and bandwidth requirements at any time, through signaling for TE-enabled traffic, or just transmitting more traffic for non-TE-enabled traffic, such as LDP traffic. This can cause traffic congestion on the MPLS-TP if some traffic is not pre-empted from the MPLS-TP LSP.